An earth-boring drill bit is typically mounted on the lower end of a drill string and is rotated by rotating the drill string at the surface or by actuation of downhole motors or turbines, or by both methods. With weight applied to the drill string, the rotating drill bit engages the formation and proceeds to form a borehole along a predetermined path toward a target zone. The borehole formed in the drilling process will have a diameter generally equal to the diameter or “gauge” of the drill bit.
A typical earth-boring bit includes one or more rotatable cutters that perform their cutting function due to the rolling movement of the cutters acting against the formation material. The cutters roll and slide upon the bottom of the borehole as the bit is rotated, the cutters thereby engaging and disengaging the formation material in its path. The rotatable cutters may be described as generally conical in shape and are therefore sometimes referred to as roller cones. Such bits typically include a bit body (12 of FIG. 17) with a plurality of journal segment legs (16 of FIG. 17). The roller cone cutters (28 of FIG. 17) are mounted on bearing pin shafts that extend downwardly and inwardly from the journal segment legs. The borehole is formed as the gouging and scraping or crushing and chipping action of the roller cones remove chips of formation material which are carried upward and out of the borehole by drilling fluid which is pumped downwardly through the drill pipe and out of the bit.
The earth-boring action of the roller cone cutters is enhanced by providing the cutters with a plurality of cutter elements. Cutter elements are generally two types: inserts formed of a very hard material, such as cemented tungsten carbide, that are press fit into undersized apertures or similarly secured in the cone surface; or teeth that are milled, cast or otherwise integrally formed from the material of the roller cone. Bits having tungsten carbide inserts are typically referred to as “TCI” bits, while those having teeth formed from the cone material are known as “steel tooth bits.” The cutter elements on the rotating cutters breakup the formation to create the new borehole by a combination of gouging and scraping or chipping and crushing,
The cost of drilling a borehole is proportional to the length of time it takes to drill to the desired depth and location. In oil and gas drilling, the time required to drill the well, in turn, is greatly affected by the number of times the drill bit must be changed in order to reach the targeted formation. This is the case because each time the bit is changed, the entire string of drill pipe, which may be miles long, must be retrieved from the borehole, section by section. Once the drill string has been retrieved and the new bit installed, the bit must be lowered to the bottom of the borehole on the drill string, which, again must be constructed section by section. As is thus obvious, this process, known as a “trip” of the drill string, requires considerable time, effort and expense. Accordingly, it is always desirable to employ drill bits which will drill faster and longer and which will remove more earth per revolution of the roller cone.
To keep costs down, it is important that the drill bit achieves the highest rate of penetration while drilling a borehole. One cause of slowed drill bit penetration is a cutting structure that allows ridges of uncut earth to build up. The uncut earth is the area on the borehole bottom that is not removed during the formation of the crater. If this uncut area is allowed to build up, it forms a ridge. In some drilling applications this ridge is never realized, because the formation material is easily fractured and the ridge tends to break off. In very soft rock formations that are not easily fractured, however, the formation yields plastically and a ridge may build up. This ridge build-up is detrimental to the cutter elements and slows the drill bit's rate of penetration. For this reason, the cutting structure arrangement must mechanically gouge away a large percentage of the hole bottom in order to drill efficiently.